Heat treating container



H. J. STEIN Dec. 7, 1937.

HEAT TREATING CONTAINER igina l Filed July 13, 1932 w B R u N O C m S Patented Dec. 7, 1937 UNITED STATES HEAT TREATING CONTAINER Harold J. Stein, Milwaukee, Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation of Delaware Continuation of application Serial No. 622,238, July 13, 1932. This application May 12, 1936,

Serial N0. 79,312

4 Claims.

This invention relates, generally, to the art of heat treating metallic articles, and it relates more specically to an improved container or like structure which may be used as an enclosure 5 for articles to be heat treated and which may be delivered into and removed from a furnace together with the articles to be heat treated. The present application is a continuation of application Serial No. 622,238, filed July 13, 1932.

In various heat treating operations such as carburizing, nitriding, malleableizing and others in which the articles to be heat treated are to be enclosed Within a container or the like and the latter, together with the articles, is delivered into a furnace and removed therefrom, the container is exposed to a number of injurious influences which limit its life to a greater or lesser number of operations, depending on how Well the container is capable of resisting these injurious influences. High temperatures which are usually required for the mentioned heat treating operations, alternate heating and cooling, chemical reactions With substances with which the container may come in contact during the heat treat-ing operations and erosive attack by gases Within and Without the container, are the most common causes for failure or destruction of the container after aV limited number of operations. On the other hand it is usually necessary for carrying out the mentioned heat treating operations in a satisfactory manner that the container be made of a material which has a good heat conductivity, high strength at elevated temperatures land which is light enough to insure a light weight of the container for easy handling. In heat treating operations which aim to effect a change of the carbon content of ferrous articles such as carburizing and decarburizing, it is particularly desirable that the container be made of a material which 40 is substantially unaffected by the chemical conditions brought about to effect the desired change of the carbon content of the articles, the said conditions usually involving the presence or crea- 45 tion of carbon monoxide gas and other gases which are likely to affect or injure the container.

It is an object of the invention to provide a container or the like for heat treating purposes which is made of a material having a high heat conductivity, low specific weight, high strength at elevated temperatures, which is not easily affected by chemical reactions with substances and gases with which it may come in contact during heat treating operations, and which will effectively resist the erosive action of gases within or without the container while the latter and its contents are being heated in a furnace.

It is a further object of the invention to reduce expenses and eliminate Waste of material from which the containers or the like are made, which expenses and Waste have heretc.

fore been incurred in various heat treating opera- L rials which have heretofore been used for con-` tainers or the like in heat treating operations. v

More specifically it is an object of the inven tion ,to provide a container for carburizing and decarburizing operations, which is substantially unaffected by the chemical conditions brought.

about in such operations.,

A still further object of the invention is to improve .the effectiveness of the gases evolved from heat treating compounds at high temperatures within a container enclosing the compounds and the articles to be heat treated'.

These and .other objects and advantages of the invention will be apparent from the following description. A clear conception of an embodiment of the invention may be had by referring to the drawing accompanying and forming a part of this specification in which like reference characters designate the same or similar parts in the several views.

Fig. l shows a section through a container to bc used in heat treating metallic articles.

Fig. 2 shows a container similar to the one shown in Fig. 1 butin position on va tray for delivery into. a furnace.

The numeral I indicates, generally, a conf tainer which may be used for heat treating purposes andwhich may be delivered into and removed from a furnace together with the articles to be heat treated. The container may be of any desired shape and the cross section shown in Fig. l may be that of a cylindrical or of a box like container. In Fig. 2 the container has been shown .as being of cylindrical shape. The essential feature of the container according to the present invention is the fact that it is made of a refractory material as distinguished from metallic materials from which similar containers for heat treating operations have heretofore been made. The material from which the container is made also has a high heat conductivity, and in this respect it is distinguished from certain re fractory materials Which are known at the present time andwhich have a low heat conductivity.

It has been found-however, that certain of the well known refractory materials having a low heat conductivity may nevertheless be used as the sole or chief ingredient of a mass having a comparatively high heat conductivity, for instance, six to seven times as high as the heat conductivity of re clay, and they may therefore be used for the contemplated purpose. Such masses consisting solely or chiefly of refractory material are well known and no claims are made herein to any such mass itself. For instance, silicon carbide in crystalline form and bonded with a ceramic bond, such as a refractory clay, is a most suitable material for containers according to the present invention, this material being comparatively cheap and having a heat conductivity about six to seven times as high as re clay. Other refractory materials, however, may be used. Generally, the refractory material will be of originally granular or crystalline structure and `suitably solidified. For purposes of forming and solidication the refractory material is either sintered or combined with an organic or inorganic plasticizer. When organic plasticizers are used they carbonize upon firing which may or may not be accompandied by sintering of the mass. When inorganic plasticizers are used the mass is preferably sintered by firing.

The container shown in Fig. 1 is adapted, as mentioned above, for heat treating purposes. If used for carburizing for instance, it is packed with the articles to be case hardened and with a suitable case hardening substance in the usual manner. A tray 2 shown in Fig. 2 is then placed on top of the container and the latter, together with the tray, is turned upside down as shown in.v Fig. 2, and in this relative position the container and the tray are delivered into a furnace.

The tray 2 is preferably also made of silicon carbide or other refractory material which will not be easily attacked by gases issuing from the case hardening substance during the heating process.

It should be noted that the container described hereinbefore, is exceptionally well adapted for heat treating purposes, and possesses outstanding advantages over the metallic containers which have heretofore been used for the same purpose. Tests and observations have shown that among the harmful effects to which metallic containers are subjected in heat treating operations the most severe are due to the .use of energizing ingredients in the heat treating compounds, such for instance as carbonates of sodium, barium and calcium which substances are known as energizers for carburizing compounds. Especially sodium carbonate, which is the most effective of the mentioned energizers, is considered to have the most detrimental effect upon metallic containers, even upon those made of high-priced alloys such as chrome-nickel. According to the present invention, the mentioned harmful effects, especially those resulting from the use of sodium containing energizers, are avoided by using a non-metallic container of the type referred to hereinbefore. A generally accepted carburizing compound consisting of to 12% barium carbonate, 2 to 3% calcium carbonate, 2 to 3% sodium carbonate, 25 to 36% coke and a balance of hardwood charcoal, will not develop, during the heating process, any harmful reactions with a container made of silicon carbide as described hereinbefore. The ability of the material, from which the container is made, to resist such chemical reactions not only insures a long life of the container but it also promotes the effectiveness of the carbon monoxide gas developed in the container, in that this gas is preserved for the purpose for which it is actually intended and not absorbed by the container.

A further advantage of the container embodying the invention should be seen in its high heat conductivity which for silicon carbide is about three times as high as that of the alloys generally in use at the present time for carburizing and similar boxes. Therefore if the containers embodying the invention are used in a counterflow furnace a rapid exchange of heat between the hot and cold containers will take place and result in a substantial saving of fuel costs. Further advantages which may be realized in a container embodyingthe invention are light specific weight of the material, the specic Vweight of silicon carbide being about one-third of that of iron and steel, highstrength at elevated temperatures, resistance to erosive attackby high velocity gases, and the use of materials which are not subject to corrosive attack of sulphur, the latter featurebeQ ing valuable in cases where an oil fired furnace is to be used for the heating process. v

The preferred material from which the containers according to the invention are made, has been mentioned hereinbefore as silicon carbide but similar materials, such as fused alumina, a

obtained. :Silicon carbide containers are also considered more suitable for the intended purpose than fused alumina containers because they can be made considerably stronger, somewhat lighter and with a better heat conductivity than containers of fused alumina can be made at the present time. a l

The material from which the containers according to the invention are made may suffer if exposed to rapid temperature changes. Such harmful effects, however, may be avoided by bringing the container and its contents up to the maximum heat treating temperature slowly and also allowing ample time for cooling. For instance, where the maximum temperature is 1700" F. a silicon carbide container will not suffer from being repeatedly heated to that temperature and cooled again to a temperature where it can safely be touched, if an hour or longer is allowed for the time of raising the temperature and the cooling period is similarly protracted over a time of an hour or longer.

A certain porosity of the material of the containers has been found not to affect their usefulness for heat treating purposes. Considering the fact that a heat treating compound within the container will issue upon heating gases at a certain rate of speed, the porosity and construction of the container should be such that seepage of the gases through the wall of the container, if

taking place, does not preclude the building up of a suitable gas pressure within the container. In a silicon carbide container for instance, the porosity of the material may be 20% and such porosity has been found not to have any detrimental effects where a cylindrical containenl inches long overall, 9 inches inside diameter, 101/2 inches outside diameter with one inch closed end was used in the described manner for carburizing purposes, and a case hardening compoundo f the composition specified hereinbefore was employed'Y From practical results obtained with a conventional metal container and with a silicon carbide container embodying the invention, it has been observed that articles embedded in a heat treating compound are aiected more effectively by the gases issuing from the compound, if the container enclosing the compound and the articles is porous rather than solid, the improved result being apparently due to the fact that in a porous container Where the gases are caused to seep under pressure through the walls of the container, a better circulation of the gases takes place than in a solid container` Where the gases can escape only between the body of the container and its cover or at some other limited area. It has been further observed that steel parts earburized in a cylindrical silicon carbide container as specified hereinbefore take on a better case than the same steel parts treated in the same manner in a similar metallic container as heretofore used. However, in cases where the porosity of the material should cause undesirable loss of gas Within the container by seepage, the container may be Washed periodically with graphite, or a wash made of silicon carbide or similar material may be used to retard seepage of gas.

It should be understood that it is not intended to limit the invention to the exact details of construction and procedure herein shown and described, for various modifications Within the scope of the claims may occur to persons skilled in the art.

It is claimed and desired to secure by Letters Patent:

1. A container for use in heat treating metallic articles with gases evolved at high temperatures from a heat treating compound in which the articles are embedded, said container being composed of granular silicon carbide, and a ceramic bond, the bonded mass having a heat conductivity substantially higher than that of re clay.

2. A container for use in heat treating metallic articles with gases evolved at high temperatures from a heat treating compound in which the articles are embedded, said container being composed of granular silicon carbide bonded with refractory clay, the bonded mass having a heat conductivity substantially higher than that of re clay.

3. A container for use in heat treating metallic articles with gases evolved at high temperatures from a heat treating compound in which the articles are embedded, said container being composed o-f bonded silicon carbide particles, the bonded mass having a heat conductivity substantially higher than that of fire clay.

4. A container for use in carburizing ferrous articles by means of gases evolved at high temperatures from a sodium containing heat treating compound in which the articles are embedded, said container being composed of bonded silicon carbide particles, the bonded mass being porous and having a heat conductivity substantially higher than that of re clay.

HAROLD J. STEIN. 

